[Elecraft] feeding a half-wave dipole with ladder line

[Ed Cole]

Bob,

Either you did not read what I wrote carefully enough, or I did a poor 
job of explaining.

First off:  I an not a HF DX chaser or HF Contest junkie.  HF is a very 
occasional activity when I want a casual chat or check in a local net.
Thus a compromise antenna for the "low bands".

The existing 80/40m dipole is a fan dipole with 80 and 40m wires.  I 
plan the new setup  to run 40m at right angles for less interaction 
between them (been there done it experience).

If all I wanted to do is 80/40 I would install the existing 1:1 balun at 
the feed and run coax to it.

But I was thinking I could use window wire (commercially made) instead 
of the 12-inch parallel wires I show in the diagram.  Whatever its 
impedance (probably like 300-ohm or 450).

By shorting the ladder line I could feed the antennas as a shortened 
vertical on either 160m or 630m where the dipole wire would add 
"top-hat" loading.  Similar to how the horizontal wire in an inverted-L 
works.

I already have my 630m loading coil to use at the base; adding a tap it 
could work at 160m.

I have vacuum relays to do the shorting.  And yes there will be radials 
that are too short on 630m (1/4 wave is 522 feet and my property is 200x 
300 foot).

My question regarded choosing a proper balun to use on 80 & 40m. 
Running 140 foot of ladder wire on diagonal to the shack defeats using 
it as a vertical.  Ideal would be a QRO ATU at the ground-end of the 
vertical wires (but I'm too cheap for that).  I have a QRO Drake Tuner 
to do the job at the shack.  I'll try out my 1:1 balun.


73, Ed - KL7UW

Date: Fri, 29 Oct 2021 18:48:36 -0500
From: Bob McGraw <rmcgraw at benlomand.net>
To: elecraft at mailman.qth.net
Subject: [Elecraft] feeding a half-wave dipole with ladder line
Message-ID: <8431998e-509b-9164-368a-1985394d87f6 at benlomand.net>
Content-Type: text/plain; charset=UTF-8; format=flowed

Remember that you are not matching the impedance of the ladder line, but
more so, matching the impedance of the antenna.?? The reason for ladder
line is the low loss property even with high SWR on the line.??? A good
quality 1:1 balun is preferred.?? See https://www.dj0ip.de/balun-stuff/

I've been using balanced fed antennas since 1960.?? I bring the balanced
line all the way to the tuner in the shack where it meets up with my
custom Guanella balun.? Regarding balanced fed antennas, they work, they
are easy to use, easy easy to keep in the air and DO NOT have TVI or RF
issues in the shack.?? Most of the information you hear is related to?
"old ham lore" and is not valid in any sense of the imagination.

Yes, the 130 ft center fed length will be very difficult to match on 40M
as the feed Z will be very high, in the order of 4K ohms, and not likely
in the range of your tuner.?? I suggest you add a 33 ft section of wire
to either side at the feed point. ? Thus a 80/40M antenna. ? Then you
will be able to work 80M, 40M, 20M, 15M, and 10M with that antenna
configuration.?? Mine does!? Works great.

If you want it to work 160M then it needs to be 256 ft in length.??
Although you might get a reasonable match on 160M with the 130 ft center
fed wire.?? It will depend somewhat on the feed line length acting as a
line section transformer.

As to 630M, that is a different animal and needs special attention.? I'd
suggest tying the feeders together and uss the antenna as a top loaded
vertical.

73

Bob, K4TAX